1. Field of the Invention
This invention relates to improved flame retardant aromatic carbonate polymers containing as a flame retardant additive certain metal oxides.
2. Brief Description of the Prior Art
Flame retardant properties are required for many plastics, especially those involved in electrical and transportation applications. A major flammability test used in electrical applications is the Underwriters Laboratory UL 94 test, to which many flammability standards refer. In this test, a vertical sample of the plastic of specified dimensions is ignited from the bottom by a specified flame burning applied for 10 seconds. Afterflame time and the presence or absence of flaming drips is observed. For many electrical applications, a rating of V0 by this test is the requirement; to achieve such a V0 rating, not only must the plastic sample have an afterflame burning time of less than 10 seconds but it must also not produce flaming drips. Otherwise, if flaming drips occur, or the afterflame burning time exceeds 10 seconds, the sample will be given a less favorable UL 94 rating and will be excluded from the more demanding electrical applications.
One widely used means for achieving high flame retardancy in carbonate polymers, such as V0 ratings by UL 94, hitherto have involved the use of high levels of additive or coreacted brominated compounds, usually used together with antimony oxide. Several difficulties result from this methodology: the additives commonly degrade physical properties of the polymer, for example, heat distortion temperature or impact strength. Bromine compounds and antimony are of some concern in regard to pollution. The presence of brominated components in a thermoplastic frequently causes mold corrosion. Upon combustion of such compositions, the smoke is often elevated by the presence of the flame retardant bromine/antimony system and the vapors contain corrosive hydrogen bromide which can damage nearby electrical circuitry. As alternatives to brominated flame retardants, phosphorus esters such as phosphonates and phosphates have also been used. These can cause some exudation problems, and also face environmental and toxicological questions.
Carbonate polymers in particular have been flame retarded with various sulfonate salts such as those polyfluorinated sulfonate salts described in U.S. Pat. No. 3,775,367 assigned to Bayer or the aromatic sulfonate salts such as those described in U.S. Pat. Nos. 3,940,366, 3,933,734, 3,948,851, 3,926,908, and 3,909,490 assigned to General Electric Company. However, such salts introduce halogen (with the problems cited above) and/or sulfur into the polymer which inherently contributes corrosive combustion products such as sulfur oxides if the polymer is involved in a large fire that overrides the flame retardant.
An alternative approach to flame retarding certain thermoplastics is to introduce high loadings of mineral hydrates which release water endothermically when exposed to a fire; examples are alumina trihydrate and magnesium hydroxide. However, such high loadings greatly compromise the physical properties of most plastics, and are particularly detrimental in carbonate resins.
It is an object of the present invention to provide inorganic nonhalogenated flame retardant additives for carbonate polymers which are effective at low levels, inexpensive, nonvolatile, noncorrosive and which pose minimal toxicological or environmental problem.
I have found that this requirement is met by flame retardant quantities of certain metal oxides. These results are surprising, in that certain dioxides, for example, titanium dioxide, which have been added to polycarbonates as pigments, as taught for example in Baron et al., U.S. No. Pat. 4,049,614 (1977), are not flame retardant. To flame retard such compositions, Baron et al. teach that that additional brominated components must be added.